Multijunction solar cells offer a path to very high conversion
efficiency, exceeding 60% in theory. Under ideal conditions, efficiency
increases monotonically with the number of junctions. In this study, we
explore technical and economic mechanisms acting on tandem solar cells.
We find that these mechanisms produce limitations that are the more
pronounced the greater the number of junction is and, hence, limit the
ideal number of junctions, as well as the corresponding efficiencies.
Spectral variations induce current losses in series-connected tandem
solar cells. For Denver, we find that these losses reduce achievable
harvesting efficiencies to 51% for non-concentrated light, and that
they restrict the ideal number of junctions to less than nine.
Independently operated solar cells suffer from optical losses with
similar consequences. Optical efficiencies of 99% restrict the ideal
number of junctions to below ten, and reduce achievable efficiencies by
more than 10%. Only architectures with a sequential cell illumination
are more resilient to these losses. Restricting available materials
reveals that a sufficiently low band gap for the bottom cell of 0.9 eV
or below is expedient to realize high efficiencies. Economic
considerations show that five junctions or less are economically ideal
for most conceivable applications.